This invention relates to an adhesive composition, and to an imaging medium comprising this adhesive composition. More particularly, this invention relates to a laminar imaging medium having improved resistance to stress-induced delamination.
The provision of images by resort to media which rely upon the generation of heat patterns is well known. Thermally imageable media are particularly advantageous inasmuch as they can be imaged without certain of the requirements attending the use of silver halide based media, such as darkroom processing and protection against ambient light. Moreover, the use of thermal imaging materials avoids the requirements of handling and disposing of silver-containing and other processing streams or effluent materials typically associated with the processing of silver halide based imaging materials.
Various methods and systems for preparing thermally generated symbols, patterns or other images have been reported. Examples of these can be found in U.S. Pat. No. 2,616,961 (issued Nov. 4, 1952 to J. Groak); in U.S. Pat. No. 3,257,942 (issued Jun. 28, 1966 to W. Ritzerfeld, et al.); in U.S. Pat. No. 3,396,401 (issued Aug. 6, 1968 to K. K. Nonomura); in U.S. Pat. No. 3,592,644 (issued Jul. 13, 1971 to M. N. Vrancken, et al.); in U.S. Pat. No. 3,632,376 (issued Jan. 4, 1972 to D. A. Newman); in U.S. Pat. No. 3,924,041 (issued Dec. 2, 1975 to M. Miyayama, et al.); in U.S. Pat. No. 4,123,578 (issued Oct. 31, 1978 to K. J. Perrington, et al.); in U.S. Pat. No. 4,157,412 (issued Jun. 5, 1979 to K. S. Deneau); in United Kingdom Patent Specification No. 1,156,996 (published Jul. 2, 1969 by Pitney-Bowes, Inc.); and in International Patent Application No. PCT/US87/03249 of M. R. Etzel (published Jun. 16, 1988, as International Publication No. WO 88/04237).
In the production of a thermally actuatable imaging material, it may be desirable and preferred that an image-forming substance be confined between a pair of sheets in the form of a laminate. Laminar thermal imaging materials are, for example, described in the aforementioned U.S. Pat. Nos. 3,924,041 and 4,157,412 and in the aforementioned International Patent Application No. PCT/US87/03249. It will be appreciated that the sheet elements of a laminar medium will afford protection of the image-forming substance against the effects of abrasion, rub-off and other physical stimuli. In addition, a laminar medium can be handled as a unitary structure, thus, obviating the requirement of bringing the respective sheets of a two-sheet imaging medium into proper position in a printer or other apparatus used for thermal imaging of the medium material.
In the aforementioned International Patent Application No. PCT/US87/03249, there are described certain preferred embodiments of a high resolution thermal imaging medium, which embodiments include a porous or particulate image-forming substance (e.g., a layer of pigment and binder) confined in a laminate structure between a pair of sheets. Upon separation of the respective sheets, after laser exposure of portions or regions of the medium, a pair of complementary images is obtained. Among the laminar embodiments of International Patent Application No. PCT/US87/03249 are those which include: a first sheet transparent to image-forming radiation and having at least a surface zone or layer of polymeric material which is heat-activatable upon subjection of the medium to brief and intense radiation; a layer of porous or particulate image-forming substance thereon; and a second sheet laminated and adhesively secured to the first sheet.
Upon exposure of regions or portions of the medium to brief and intense image-forming radiation, and conversion of absorbed energy to heat for activation of the heat-activatable polymeric material, corresponding regions or portions of the image-forming substance are caused to be more firmly attached or locked to the first sheet. Abutting regions or portions of image-forming substance which are not subjected to such image-forming radiation are, upon separation of the first and second sheets, removed by the adhesive second sheet, for formation of an image complementary to the image on the first sheet. In preferred thermal imaging media of the aforementioned International Application, a release layer is provided over the porous or particulate image-forming substance to facilitate proper separation of the respective first and second sheets and formation of the respective complementary images.
The respective images obtained by separating the sheets of an exposed thermal imaging medium having an image-forming substance confined therebetween, such as a laminar image medium of the type described in the aforementioned International Application, may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of and nature of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image. The principal image may, however, depending upon the aforementioned properties and modes of failure, exhibit decidedly inferior properties, such as poorer handling characteristics, durability and abrasion resistance, as compared with the complementary image of secondary importance.
In the production of thermal images from media having "first" and "second" sheets, of the type described in the aforementioned International Application, it will oftentimes be preferred, in the case of high density images, that the principal image be that which is formed on the second sheet by transfer of non-exposed regions of coated image-forming substance. It will be recognized that an alternative is to form a high density image on the first (opposed) sheet by firmly attaching the image-forming substance in areas of exposure. This is the case because the medium provides complementary images and the desired high density image can be formed on either sheet by addressing the thermally actuatable medium according to which sheet shall bear the high density image. Formation of a high density image on the first sheet is, however, disadvantageous since the areas of high density are created in areas of exposure (by activation of a heat-activatable image-forming zone or layer) and large areas of image-forming substance require correspondingly large areas of laser actuation and energy utilization and highly accurate laser scanning and tracking. Errors in tracking will result in discontinuities (whiteness or voids) by failure to attach minute regions of image-forming substance and by their removal to the opposed (second) sheet upon separation of the sheets. Owing to the psychophysical nature of human vision, minute regions of lightness (voids) against an expansive darkness tend to be noticeable.
It will, thus, be preferred that a high density image be the result of the transfer in non-exposed regions of coated and continuous regions of image-forming material (with minimal or no discontinuities or coverage voids), rather than the result of firm connection of high density regions of imaging material by laser-actuated operation of the heat-activatable image-forming surface, where tracking errors increase the possibility of creating noticeable areas of discontinuity (whiteness) against the expansive high density region.
Inasmuch as the formation of a preferred image in non-exposed portions of image-forming substance will be the result of the removal of such substance from an opposed sheet with the aid of an adhesive sheet, the adhesive thereof will serve as a base for the image carried by the sheet. The nature of the adhesive, and especially its physical properties, may influence image quality and certain physical attributes of the image, such as the handling properties and durability of the image. If the wrong adhesive is used, the laminar medium material may exhibit an undesirable tendency to delaminate upon subjection to certain physical stresses that may be created during a manufacturing operation (e.g., bending, winding, cutting or stamping operations). It may be desirable in some instances to form a laminar medium from a pair of endless sheet or web materials and to then cut, slit or otherwise provide therefrom individual film units of predetermined size. A reciprocal curing and stamping operation used for the cutting of individual film units may create stress influences in the medium, causing the sheets to separate at the interface of weakest adhesivity--typically, at the interface where, by thermal actuation, the preferential adhesion of the image-forming substance would be reversed.
In the aforementioned copending application Ser. No. 07/616,853, and in the corresponding International Patent Application No. PCT/US91/08585 (Publication No. WO 92/09441), there is disclosed an improved thermal imaging medium including a polymeric hardenable adhesive layer which in its unhardened condition serves to laminate the sheets of the medium into a unitary medium having a reduced tendency to delaminate upon subjection to physical stresses and which, upon subsequent hardening (curing), provides sufficient hardness to provide improvements in image handling and durability; thus, the hardenable adhesive provides a first degree of adhesion (hereinafter referred to as "pre-curing adhesion") when the two sheets are contacted with one another or shortly thereafter and a second degree of adhesion (hereinafter referred to as "post-curing adhesion") after the adhesive is cured. A preferred type of hardenable adhesive for use in this medium comprises a macromolecular organic binder; a photopolymerizable ethylenically unsaturated monomer having at least one terminal ethylenic group capable of forming a high molecular weight polymer by free radical-initiated, chain-propagated addition polymerization; and a free radical-generating, addition polymerization-initiating system activatable by actinic radiation.
This preferred type of hardenable adhesive gives good results. However, the preferred hardenable adhesive formulations described in the aforementioned application Ser. No. 07/616,853, which comprise a polyfunctional acrylate monomer admixed with a methacrylate copolymer, require a substantial lag time (the period between the lamination of the two sheets and the curing of the hardenable adhesive) to ensure that after curing the two sheets adhere sufficiently to one another. This substantial lag time, which is of the order of tens of minutes, is presumably required because it is necessary for the polyfunctional acrylate monomer to diffuse into an adjacent layer of the imaging medium in order to provide sufficient post-curing adhesion. In some cases, as for example where it is desired to carry out curing of the adhesive "in line" with the lamination (i.e, when the medium is to move continuously at a substantial speed along a production line from the lamination to the curing operations, perhaps via intervening cutting or other stations), the need for a substantial lag time in order to develop post-curing adhesion is disadvantageous since the production line must be modified to provide a long travel for the medium between the lamination station and the curing station, and providing such a long travel will normally involve the provision of numerous extra rollers in the production line, thus increasing the cost, size and power consumption of the line.
It has now been found that, by modifying the adhesive compositions described in the aforementioned application Ser. No. 07/616,853, and International Patent Application No. PCT/US91/08585, the lag time necessary to develop substantial post-curing adhesion can be substantially reduced.